1. Field of the Invention
The present invention relates to a display skill field, and more particularly to a scan driving circuit for the oxide semiconductor thin film transistors.
2. The Related Arts
GOA (Gate Drive On Array) is to manufacture the gate driver on the array substrate by utilizing the thin film transistor (TFT) liquid crystal display array process for achieving the driving method of scanning line by line.
Generally, the GOA circuit comprises a pull-up part, a pull-up controlling part, a transfer part, a pull-down part, a pull-down holding part and a boost part in charge of boosting voltage level. The boost part generally comprises a bootstrap capacitor.
The pull-up part is mainly in charge of outputting the inputted clock signal (Clock) to the gate of the thin film transistor as being the driving signals of the liquid crystal display. The pull-up control part is mainly in charge of activating the pull-up part, and is generally functioned by the signal transferred from the former GOA circuit. The pull-down part is mainly in charge of rapidly pulling down the scan signal (i.e. the voltage level of the gate of the thin film transistor) to be low voltage level after outputting the scanning signal. The pull-down holding circuit part is mainly in charge of maintaining the scanning signal and the signal of the pull-up part in an off state (i.e. the set negative voltage level). The boost part in mainly in charge of performing a second boost to the voltage level of the pull-up part for ensuring the normal output of the pull-up part.
With the development of the oxide semiconductor thin film transistor, the peripheral circuit around the panel corresponding to the oxide semiconductor also becomes the focus that people pay lots of attentions. Because the carrier mobility of the oxide semiconductor thin film transistor is 20-30 times of the amorphous silicon thin film transistor, which is capable of magnificently raising the charging/discharging rate of TFT to the pixel electrodes to promote the response speed of the pixels and to realize faster refreshing rate. In the mean time, the line scan rate of the pixels also can be significantly promoted to make the production of the flat panel display with ultra high resolution possible. Therefore, the GOA circuit of oxide semiconductor thin film transistor has potential to replace the GOA circuit of amorphous silicon thin film transistor. In prior arts, only a few developments are proceeded for focusing on the GOA circuit of oxide semiconductor thin film transistor. Particularly, many problems caused by the electrical property of the oxide thin film transistors themselves have to be overcome. For instance, threshold voltage is generally larger than 0V in the electrical property of the traditional amorphous silicon thin film transistor and the swing of the subthreshold range voltage is relatively larger than the electrical current. Therefore, in the design of the circuit, the generated leakage current is smaller even when the voltage Vgs between the gate and the source is about 0V in some transistors operations. However, the threshold voltage of the oxide semiconductor thin film transistor is about 0V and the subthreshold range swing is smaller because the material property thereof has significant differences from the amorphous silicon. Many elements may function at Vgs=0V when the GOA circuit is in an off state. Accordingly, the design difficulty of a GOA circuit for the oxide semiconductor thin film transistors will be increased. There will be some function issues happening when the design adaptable to the scan driving circuit for the amorphous silicon semiconductors is applied to the oxide semiconductor. Besides, due to some external factor inductions and the stress effect, there will be a tendency that the threshold voltage diminishes toward minus value to the oxide semiconductor thin film transistor, which may directly results in malfunction of the GOA circuit for the oxide semiconductor thin film transistors. Therefore, the influence of the element property to the GOA circuit has to be considered as designing the circuit.